If you live in a mixed environment of Windows 7 and 8/10 computers, you might want to set the usernames to be the same so you can share the files/printers without managing Homegroup.

Nonetheless, in Windows 7/2008, if you try to create a user account using the traditional tool (Local Users and Groups) in Computer Management, you are not allowed to use email address as user name because they banned the at-mark (@):

I searched the web for quite a while, came across stuff like UPN (User Principal Name) without luck (No active directory on Windows 7, nor I want to setup a domain controller in Windows 2008 for home network). Turns out the solution is dead simple: use the “User Account” from Control panel to create the user account. No questions asked!